The present invention concerns a method of manufacturing a semiconductor device and, more in particular, it relates to a method of manufacturing a field effect transistor (hereinafter referred to as MOSFET) using a film of transition metal oxide as a gate insulating film.
In manufacturing a MOSFET, after patterning a gate electrode and a gate insulating film into a predetermined shape, the surface of a substrate or polysilicon gate electrode has been slightly oxidized before the formation of source and drain as is well-known in the prior art.
Since edges of the gate insulating film suffers from damages caused by etching, the above-mentioned oxidation step is applied for recovering the damage and prevent the occurrence of leak current and failure of voltage withstanding and this is an indispensable step, being referred to as "light oxidation".
However, if a film of transition metal oxide such as a tantalum pentoxide film is used as the gate insulating film, the rate of oxygen diffusing in the film of such material is much greater than that in silicon dioxide used so far. Accordingly, when a tantalum pentoxide film 2, a silicon dioxide film 3, a tungsten film 4 and a PSG (phosphosilicate glass) film 5 are formed in lamination on a silicon substrate 1 applied with patterning and then light oxidation as shown in FIG. 1a, oxygen diffuses from the exposed edges to the inside of the gate insulating films 2,3 to oxidize the semiconductor substrate 1 and the gate electrode 4 present above and below thereof respectively. As a result, a wedge-like oxide layer 101' is formed at the edge of the gate region as shown in FIG. 1b. This phenomenon is remarkable in a case where streams are contained in an oxidizing atmosphere upon applying light oxidation. As a result, this causes a problem that an inversion voltage is increased in a channel region at a portion in which the wedge-like oxide layer 101' is formed to increase a threshold voltage. It is difficult to completely prevent this phenomenon even by forming an oxide film 5' on the side wall of the gate 2 as shown in FIG. 1c.
It has further been found that when the transition metal oxide film 2 as the gate insulating film is patterned simultaneously with the formation of the gate electrode 4, a leak current tends to flow through the edge of the gate insulating film 2. Further, as shown in FIG. 2a, if oxidation is applied in a state where a gate insulating film 23 is exposed at the outside of a gate electrode 24, oxygen diffuses from the exposed portion to the inside of the gate insulating film 23 to oxidize a semiconductor substrate 21 or the gate electrode 24 present below and above thereof to form wedge-like oxide layers 28, 28' at the end of the gate region as shown in FIG. 2b. This results in a problem that an inversion voltage is increased at a channel region of MOSFET in a portion where the wedge-like oxide layers 28, 28' are formed to increase the threshold voltage.
In FIGS. 2a, 2b, denoted at 22 is a thick insulating film for inter-device isolation and at 23' an insulating film formed by the oxidation of the surface of the gate electrode 24.
In recent years, the width of the gate electrode in MOSFET has been narrowed extremely and a MOSFET having a gate electrode with width of 0.2 .mu.m or less has been proposed. Then, if wedge-like oxide layers should be formed below such a fine gate electrode, it is apparent that the operation as the MOSFET is scarcely possible.